One of the most popular politicians in Mexico, Mr Lopez Obrador, 52, was very nearly stopped from running in the presidential elections. In 2005, an attempt was made to derail his bid for the presidency by impeaching him over a minor land dispute.

The ensuing furore led hundreds of thousands to march in his support in Mexico City. It ended in a triumph for him when the legal action was stopped and President Vicente Fox sacked the attorney-general.

As mayor of Mexico City, he won respect as much for his reputation for honesty, a gruelling work schedule and his humble lifestyle as for his ambitious public works and social programmes targeting the poor and disadvantaged.

He often draws on his humble origins. The son of a store owner, he grew up in a village of 600 in Tabasco State, and has been seen as the candidate of the common man and woman.

His built his campaign around "50 Commitments To Rebuild National Pride", which stressed the recognition of indigenous people's rights, scholarships for the handicapped and improving healthcare and education.

In office, he said he would pay for social spending, higher pensions and wages by wiping out corruption, cut down on government waste and crack down on tax evasion. His ran his campaign with the slogan: "For the good of all, the poor first."

His anti-capitalist speeches sent jitters through the business community and his main rival said he would bankrupt the country. However, Mr Lopez Obrador, insisted he would respect private property and foreign business investment.

The US watched the election carefully to see if the left-wing candidate would be a moderate like Brazil's President Lula or a firebrand socialist like Venezuela's Hugo Chavez.

Felipe Calderon - National Action Party (PAN)

Felipe Calderon: Tough talk on crime

A Harvard educated lawyer, Mr Calderon, 44, found favour with the business community and has pledged to continue the free market policies pursued by President Vicente Fox.

A career politician, he served as head of the national development bank, Banobras, under Mr Fox and was energy secretary from 2002 to 2004. He is also a former president of the PAN, the party his father, Luis Calderon Vega, helped to create.

He ran a negative campaign against his left-wing rival, linking him to Venezuela's President Hugo Chavez in TV ads, proclaiming: "Lopez Obrador is a danger to Mexico".

The Federal Electoral Institute (IFE) eventually banned the ad, despite Mr Calderon's claim that the move constituted censorship. Mr Calderon was stung by accusations from Mr Lopez Obrador that he gave contracts to a company part-owned by his brother-in-law while he was energy secretary. Mr Calderon strongly denied the allegations.

While Mr Lopez Obrador has pledged to fight crime through social programmes, Mr Calderon has pledged an iron fist approach, with life sentences for kidnappers. He also said he would push for a migration agreement with the US to legalise residency for Mexicans who have lived there for over three years.

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